1. Field of the Invention
The present invention relates to a gas sensor that has a sensor element and measures a predetermined gas component in a measurement gas. The invention particularly relates to a gas sensor that has a protective cover arranged so as to surround a sensor element.
2. Description of the Background Art
Conventionally, various measuring devices are used to know the concentration of a desired gas component in a measurement gas. For example, as devices that measure a NOx concentration in a measurement gas such as a combustion gas, gas sensors are publicly known that have an electrochemical pump cell structured by forming a Pt electrode and an Rh electrode on a solid electrolyte layer made of zirconia (ZrO2) having oxygen ion conductivity.
Such gas sensors are used for measuring a desired gas component included in exhaust gases from automobiles. The gas sensors to be mounted into tail-pipes of automobiles are provided with protective covers for protecting sensor elements in order to mainly prevent adhesion of water generated at the time of engine starting to the sensor elements and prevent water intrusion into the sensor elements.
Such various protective covers are already publicly known. For example, a protective cover, in which a flow of measurement gases is a swirl flow in one direction, is publicly known (for example, refer to Japanese Patent Application Laid-Open No. 01-296159 (1989)). Further, a protective cover is also publicly known that represses occurrence of a crack on a sensor element due to the adhesion of water and a decrease in temperature of the sensor element due to a decrease in temperature of measurement gases (for example, refer to Japanese Patent Application Laid-Open No. 10-318980 (1998)).
Further, protective covers are also publicly known that adopt a triplex structure (an inner protective cover, an intermediate protective cover, and an outer protective cover) so as to effectively prevent the adhesion of water (for example, refer to Japanese Patent Application Laid-Open Nos. 2000-304719 and 2004-301579).
It is necessary for heightening the accuracy of gas sensors to heighten responsiveness, namely, enable the measurement of concentration quickly conforming with a concentration change of a predetermined gas component in a measurement gas and to reduce the adhesion of water to a sensor element generated at the time of engine starting.
In the case of the gas sensors having the protective covers, it is necessary for securing the responsiveness that atmospheres in the protective covers are quickly replaced by measurement gases present outside the protective covers.
However, to heighten a replacement property of the measurement gases in the protective covers conflicts with to reduce the adhesion of water to the sensor elements, and thus it is difficult to simultaneously realize both of them.
The protective covers disclosed in Japanese Patent Application Laid-Open Nos. 01-296159 and 10-318980 have the structure such that the reduction in the adhesion of water and the intrusion of water into the sensor elements is emphasized. For this reason, their responsiveness is not sufficient.
An object of the protective covers adopting the triplex structure disclosed in Japanese Patent Application Laid-Open No. 2000-301749 and Japanese Patent Application Laid-Open No. 2004-301579 is to effectively prevent the adhesion of water and simultaneously obtain high responsiveness. However, since the intermediate protective covers are provided, manufacturing steps become complicated and thus production cost becomes high.